There are several known techniques to date for the watermarking of video sequences.
One of these techniques consists of the application, to the video sequences, of watermarking methods similar to those used for the watermarking of still images. Indeed, it can be assumed that a video stream consists of a sequence of still images and that the “intra” images of the sequence can therefore be watermarked according to a scheme conventionally applied to the still images.
However, video sequences have many properties that can be advantageously exploited to develop a watermarking technique that is proper to them.
These properties include the following aspects:                the rough size of a video sequence is far greater than that of a still image, so that the insertion space of a mark is far greater in a video sequence;        as compared with still images, video sequences have a time dimension, which can be used for the insertion of the mark.        
Furthermore, video sequences have constraints different from those of still images. Thus:                the complexity of the watermarking scheme of a video sequence must be low enough to enable the insertion and detection of a mark to be done on the fly;        the mark of a video sequence must be less visible than in the case of a still image, because the motion of the objects often increases its perceptibility to the user;        the video streams are often compressed in order to reduce the rough size of the sequences. The compression standard most commonly used is the MPEG2 standard, and the mark can be inserted directly into the format itself. However, insertion into the decompressed format should not result in an increase in file size after compression;        video sequences can undergo more attacks than still images, and this often leads to the insertion of the mark or watermark redundantly so as to reduce the chances of success of these attacks, and make the watermark more robust. When the watermark is inserted redundantly in the sequence, it may be estimated through a computation of an average value on all the images of the sequence. An additional constraint therefore is that it should be possible to detect the watermark or signature even after a loss of synchronization produced by the selection of a precise sub-sequence of the video sequence considered, or by the loss of images of the sequence.        
In addition to watermarking techniques from the field of still images, watermarking techniques proper to video have also been developed. These techniques make use of the advantageous properties of video sequences, in taking account of the constraints associated with them.
Among watermarking techniques specific to video, several approaches relying on the insertion of a mark into motion vectors have been proposed, especially by F. Jordan, M. Kutter, T. Ebrahimi in “Proposal of a watermarking technique for hiding/retrieving data in compressed and decompressed video”, ISO/IEC document JTC1/SC29/WG11/MPEG97/M2281, July 1997, and by J. Zhang, J. Li, L. Zhang in “Video watermarking technique in motion vector”, Proc. XIV Brazilian symposium on computer graphics and image processing, 15-18.10.2001, pp 179-182.
F. Jordan et al. have proposed a marking algorithm in which the mark is inserted into the motion vectors of a video. This technique relies on the implementation of the following steps:                a first step for the generation of the mark, which is a binary sequence having a length of 16 bits or 32 bits;        a second step for the insertion of the mark. To do this, the motion vectors are extracted from the compressed video stream, obtained by an MPEG4 codec (or coder-decoder). If the video stream is decompressed, there is a preliminary step of compression of this stream. The different marking steps are applied to one of the two components of the motion vectors. One block of pixels per image is selected randomly and its motion vector is computed. Two bits of the mark are inserted into each component of the motion vector.        If V denotes the vertical component of a motion vector, and if b={0,1} is the value of the bit to be concealed, then the following is the algorithm for the insertion of b into V:if ((V*q+T)mod [2]≠b         
then V′=V+δ
else V′=V
with T=2*dim
dim=size of the search window for the motion estimationδ=(2·n+1)/q 
n=1 if the motion vector is the null vector.
n=0 else
q=factor of modulation of the amplitude of the motion vector
V′ is the marked motion vector.                In other words, as many motion vectors are selected as there are bits in the mark, namely 16 or 32. Then, for each of the 16 or 32 vectors selected, the corresponding bit of the mark is inserted into one of the components of the motion vector, for example the vertical component, in modifying its parity.        a third step for the extraction of the mark, during which the motion vector marked V′ is extracted from the compressed stream. The mark is then extracted as follows:b=(V′*q+T)mod [2]        
J. Zhang et al. propose an improvement of the above method proposed by F. Jordan et al., but their approach nevertheless remains the same.
These two techniques therefore rely on the implementation of a watermarking algorithm, based on the insertion of a mark within the motion vectors of a video.
However, a major drawback of these prior art techniques is that they do not have sufficient robustness with respect to possible attacks against video sequences. In particular, such methods are not sufficiently robust against non-malicious attacks such as compression or (spatial or temporal) changes in the format of the video signal.
Indeed, the above techniques proposed by F. Jordan and al. or by J. Zhang et al. rely on the implementation of a watermarking algorithm, based on the insertion of a mark into the motion vectors of a video sequence, using the notion of parity for the ordinate value of these motion vectors.
This approach is by definition not robust since the smallest attack can convert an even-parity ordinate value into an odd-parity ordinate value and vice versa.
Furthermore, according to these techniques, it is generally always the same predetermined component of the selected motion vectors (for example the vertical component) that is modified. This has the effect of increasing the visibility of the mark in the video sequence. The visibility of the mark is of course disagreeable to the user who perceives a deformation of the image.
In order to overcome these different drawbacks, the inventors of the present patent application have developed a new more robust technique for the watermarking of a video sequence described in the French patent application No. FR 02 13660 dated 31 Oct. 2002, filed by the present applicant and entitled “Procédé de tatouage d'un signal vidéo, système, support de données pour la mise en œuvre de ce procédé, procédé d'extraction du tatouage d'un signal vidéo, système pour la mise en œuvre de ce procédé” (Method for the watermarking of a video signal, system, data carrier for the implementation of this method, method for the extraction, from the watermark, of a video signal, system for the implementation of this method)
According to this technique, a motion vector that will carry the watermark is selected. The coordinates of the selected motion vector are identified in a space comprising a first plurality of zones associated with the binary value 1 and a second plurality of zones associated with the binary value 0. Then, if necessary, the coordinates of the selected motion vector are modified so that it is located in the zone in which the binary value corresponds to the value of the mark to be inserted into the selected motion vector.
Thus, if it is desired to insert a mark with a value 0, and if the selected motion vector is located in a zone associated with the binary value 1, the coordinates of the motion vector are modified by obtaining a weighted central or axial symmetry relative to the edges of the zone.
This technique proves to be therefore more robust than the methods proposed by F. Jordan and al. or by J. Zhang et al., because it defines the limited space in which the end of the motion vectors is located (zones with a binary value 1 or 0). The watermarking made therefore no longer relies solely on a change in parity of the component of a motion vector but on the shift of the vector to be watermarked over a relatively large zone.
As a consequence, even in the case of the filtering or light conversion of the image, leading to a modification of the coordinates of the watermarked motion vector, the end of this vector remains within the zone of the binary value chosen as a function of the mark. The mark is therefore robust against such attacks. However, although it is more robust, this technique is not optimal in terms of the invisibility of the mark. Indeed, the marked motion vectors cause deformation in the image which is sometimes troublesome for the user.
Furthermore, the robustness of this technique against more common attacks such as compression, cropping or mirror type attacks for example is not sufficient or could be improved.